1. Field of the Invention
The present invention relates to recording layer compositions for use in optical recording media, and preferably recording layer compositions for use in recordable compact disc (CD-R) media and recordable digital videodisc (DVD-R) media. In particular, these recording layer compositions contain the combination of selected cyanine dyes with at least one selected leuco compound. This combination has been found to provide enhanced dye solution stability and dye layer photostability, as well as provide good recording properties. Furthermore, the present invention relates to substrates coated with these recording layer compositions, as well as recordable medium including such coated substrates.
2. Brief Description of the Art
Several technologies have emerged in the writable optical recording media art. These include the so-called "Write Once Read Many" (WORM) recording media, the CD-Recordable (CD-R) recording media and the recordable digital videodisc (DVD-R) recording media.
In WORM recording media, information is recorded onto the recording layer in the form of a pit "burned in" by an incident laser beam. This laser burning operation involves transient temperatures of above 1000.degree. C., which causes ablation of the recording media and forms the desired pit. The recording layer in WORM recording media may be various materials such as metal films, organic dye films or mixtures of dyes and polymers in film form. Reproduction of this information thus stored in a WORM recording media is generally accomplished by irradiating the media with another laser beam that has a weaker output than for recording. The irradiation differences (or contrast) between the pitted areas and the non-pitted areas on the recording media are read as electric signals, which are then translated into useful information.
CD-R recording media have been developed as an alternative to WORM media. Because CD-R media may be used with the widely available read-only CD-ROM audio and computer equipment, it has been favored over WORM for many commercial applications. CD-R media generally utilize a recording layer of organic dye positioned between a substrate and a reflective layer. Like WORM media, information is reproduced onto the CD-R recording layer by means of a laser beam. However, the transient temperature rise resulting from the absorption of radiation of this incident laser beam is far less in CD-R recordings than with WORM recordings. Typically, CD-R media experience only a 200-300.degree. C. transient temperature rise. In CD-R recording, the resulting organic dye recording layer decomposition in combination with the concurrent thermal diffusion of both the dye and the substrate, along with deformation of the reflecting layer, causes local changes in the optical properties of CD-R media that result in an adequate signal-to-noise ratio (SNR) when played with a second, but weaker, laser beam. In both WORM and CD-R recording, the laser wavelength is typically in the range 770-830 nm and the recorded spot size is about 0.80 .mu.m.
Besides the presence of the reflective layer that forms an optical interface with the recording layer, CD-R media differ from the WORM media in at least one other important way. In order to attain the desired high (65-70%) reflectivity needed to meet CD-R standards, as set forth in the Philips and Sony Orange Book Part 2 (CD-R Standards version 2.0, published by Philips Consumer Electronics BV, November, 1994), the CD-R recording layer has a lower absorption for light at the recording wavelength. In contrast, WORM technology employs a higher optical recording absorption layer.
DVD-R has recently been introduced as an improvement to both WORM and CD-R technologies. DVD-R recording processes allow for greater density of data to be stored on a storage disc having the same storage area. This is achieved by reducing the laser wavelength and the recording spot size. In DVD recording systems, the recording laser wavelength is around the 630-650 nm range and the spot size is about 0.40 .mu.m.
Regardless of the type of recording system, the wavelengths suitable for the recording and reproduction of optical data are limited by the availability of reliable, compact lasers. This is especially true for consumer applications where the lasers must also be low cost, diode type which have a limited selection of output wavelengths. For this reason, these types of recording media must be capable of being adjustable with respect to optical properties such as absorption and reflectivity in order to tune their response to particular laser wavelengths.
In WORM, CD-R and DVD-R media, the dye recording layer is generally formed on a pre-grooved substrate (e.g., an injection molded disc of polycarbonate). The grooves are present to facilitate the guiding of the laser beam both on recording and upon subsequent reproduction of the recorded signal. The dye recording layer is deposited by spin-coating a solution of the dye components in such a manner as to produce a radially uniform thickness deposited both within the grooves and on the adjacent lands between the grooves.
For a typical optical recording medium the average film thickness of the dye layer over the lands and grooves of the substrate is in the range 50 to 160 nm, and preferably for CD-R is typically in the range of 70 to 130 nm and preferably for DVD-R is typically in the range of 50 to 120 nm.
The compounds used in WORM, CD-R and DVD-R optical data recording layers generally include certain classes of organic dyes in which the optical absorption characteristics may be manipulated to achieve the best overall combination of functional characteristics, such as write sensitivity, read stability, solubility in coating solvent, shelf life, and other properties known to those skilled in the art. These organic dyes also offer low thermal conductivity which results in low heat loss and higher writing sensitivity (reduced laser power) compared with other highly absorbing materials such as metal films. The preferred classes of organic dyes used in many optical data recording layers are cyanine dyes, particularly indo and benzindo carbocyanine dyes. Structures of particular examples of these cyanine dyes are shown in formulae (I), (II), and (III) below: ##STR1## wherein X.sup.- is a selected counteranion and R.sub.1 and R.sub.2 are alkyl groups having from 1 to 4 carbon atoms and (CH.dbd.CH).sub.n represents a polymethine chain where the number of polymethine units n=1-3. These cyanine dyes have been widely used in the manufacture of recordable optical discs. Other advantages of these classes of cyanine dyes in optical data recording applications are the relative ease of synthesis and the acceptable thermal and photochemical stability.
The wavelength region of maximum light absorption in any particular cyanine dye (or the intensity of absorption at a given wavelength) depends on the structure of the organic cation and in particular on the length of the methine chain which joins the indole or benzindole moiety and is defined by n in the above formulae. Thus, by varying n, the wavelength sensitivity to a particular recording laser wavelength may be altered. For WORM media, for which a higher absorption at the same recording wavelengths is required compared with CD-R or DVD-R media, n is usually 3. For CD-R media, having a recording wavelength in the 770-790 nm range, n is typically 2. For DVD-R media having a recording laser wavelength in the range 630-650 nm, n is typically 1. In compositions where it is advantageous to finely adjust the optical properties of the dye layer, a mixture of cyanine dye components is often preferred, in which each cyanine dye component may have a different value of n.
The use of cyanine dyes alone or in combination with other ingredients as a recording layer has been described in many U.S. patents. For example, see U.S. Pat. No. 4,600,625 (Ricoh); U.S. Pat. No. 4,656,121 (Ricoh); U.S. Pat. No. 4,923,390 (Canon); U.S. Pat. No. 4,944,967 (Fuji Photo Film); U.S. Pat. No. 5,155,009 (Pioneer); U.S. Pat. No. 5,161,150 (TDK); U.S. Pat. No. 5,213,955 (Taiyo Yuden); U.S. Pat. No. 5,275,925 (Canon); U.S. Pat. No. 5,316,184 (Fuji Photo Film); U.S. Pat. No. 5,318,882 (Taiyo Yuden); U.S. Pat. No. 5,328,741 (Pioneer); U.S. Pat. No. 5,328,802 (Pioneer); U.S. Pat. No. 5,336,584 (Pioneer); U.S. Pat. No. 5,482,822 (Canon); U.S. Pat. No. 5,512,416 (TDK); and U.S. Pat. No. 5,605,732 (Canon). The majority of these patents are directed to dye compositions having certain improvements with regard to (1) solubility in coating solvents which do not damage the substrate; (2) stability against thermal oxidation and photooxidation; (3) reduced dependence of the recording characteristics on wavelength of the laser light; (4) higher recording sensitivity; and (5) higher recording and read-out stability. Past improvements include combinations of one or more cyanine dyes with stabilizers, often referred to as quenchers. Quenchers based on transition metal complexes especially dithiol complexes of Ni, Co, Mn or Cu, such as described in U.S. Pat. No. 5,512,416 assigned to TDK Corporation are especially useful in stabilizing cyanine dyes against photochemical and thermal oxidation.
Additionally, certain types of aromatic tertiary amines capable of forming stable radicals are widely used to stabilize cyanine dyes. These may be optionally combined with a metal-dithiol complex. For example, U.S. Pat. No. 4,656,121 assigned to Ricoh Company Ltd. discloses the cationic form of particular aminium dyes used in conjunction with selected cyanine dyes suitable for WORM recording media.
Other classes of compounds, which can be used as light stabilizers, include nitrosoaniline derivatives, nitrosophenols, nitrosodiphenylamines and 1-pycryl-2,2-diarylhydrazyl free radicals as disclosed for example, in U.S. Pat. No. 5,318,882 assigned to Taiyo Yuden Co., Ltd.
Preferred compounds of the amine type, in the cationic aminium form, which absorb strongly in the near infra red, such as those shown in formulae (IV) and (V), below, have been found to be particularly suitable. These structures are as follows: ##STR2## where each R= ##STR3## and wherein each R.sub.1 is individually selected from an alkyl group having a number of carbon atoms from 2 to 5 and X.sup.- is a counteranion; and ##STR4## where each R.sub.2 is an alkyl group having a number of carbon atoms from 2 to 5.
U.S. patent application Ser. No. 08/869,870 filed Jun. 2, 1997 discloses combinations of cyanine dyes and aminium near infra red absorbing dyes shown in the above formulae (IV) and (V) in which the anion is hexafluoroantimonate and which provides additional benefits of improved resistance to variation of the recording wavelength and excellent photostability as measured on the solid films cast from the dye solution mixtures.
Stabilizers of the types described in formulae (IV) and (V) above absorb light usually in the near infra red range of the visible spectrum. While this absorption in the case of certain of these compounds such as the aminium type, may be used to advantage to reduce the wavelength dependence of the absorption of certain cyanine dyes alone as disclosed in U.S. patent application Ser. No. 08/746,449 filed Nov. 8, 1996, the changes thus produced may limit the amount of stabilizer which can be added to the formulation to provide adequate photostability of the dye recording layer. This arises because too high a concentration of the stabilizer may result in too high an absorption at the recording wavelength and the resulting disc will not achieve the high reflectivity of &gt;65% as set forth in the Orange Book Part 2 ver 2.0.
In addition to the photo stability required for the dye recording layer, it is also important that a solution of the dye used to deposit the recording layer exhibits high solution stability.
For example, the economic production of optical discs requires that the dye solution can be recovered and reused after the initial spin coating process. During the spin-coating, only a small amount of the dye solution, approximately less than 0.5%, is actually deposited upon the substrate. The remaining (spent) dye solution is spun off and collected into a bowl or a cup, from which it can be drained into a receiving vessel. Ideally, the spent dye solution should be reusable after making an adjustment to the composition to allow for solvent losses occurring during evaporation. This process is referred to as recycling and enables a much larger number of discs to be produced from the original dye solution. During the spin-coating process, solvent loss occurs by evaporation, which in principle can easily be adjusted for by the addition of fresh solvent, in an amount calculated to restore the original concentration of the dissolved components. However, changes in composition of the dye solution occurring either on storage or during use, which result in a loss of one or more solid components by crystallization for example, or the formation of decomposition products of the components, make it much more difficult to reuse or recycle the dye solution, and thus the cost of manufacture can be prohibitively high.
Thus, the practicality of such a recycling process and in particular the ability to repeat the process after successive spin-coat cycles depends upon the solution stability of the formulated dye components.
In addition to the need for a stable dye recording layer and a stable solution, the solubility of the dye components and the stabilizer must be sufficiently high so that solutions suitable for depositing a thin film of the recording dye layer can be easily prepared.
For example, U.S. patent application Ser. No. 08/869,870 filed Jun. 2, 1997 discloses the use of cyanine dyes having certain anions which exhibit excellent solubility in ethyl lactate, a safe solvent which has desirable characteristics for spin coating on polycarbonate substrate, and a fairly rapid evaporation rate which results in a reduced cycle time for the coating process hence higher thruput for finished discs.
In the presence of an aminium dye stabilizer such as those shown in formulae (IV) and (V), or in the presence of combinations of an aminium dye stabilizer and a nickel, copper, manganese or cobalt dithiolato stabilizer, such cyanine dyes exhibit excellent stability against decomposition due to oxidation.
In many cases, stabilizers of the types described in the forgoing U.S. patents exhibit limited solubility in selected coating solvents which do not attack polycarbonate such as diacetone alcohol, ethyl lactate, tetrafluoropropanol and octafluoropentanol.
Furthermore, the aminium dye stabilizers, such as those shown in formulas (IV) and (V), may undergo decomposition in solution, which causes composition changes to occur over time which adversely affects the ability to recycle the dye solution. Moreover, a reduction in the concentration of the aminium dye stabilizer resulting from decomposition changes in the optical absorption of the recording layer produced upon spin-coating the solution, and may also reduce the ability of the recording layer to resist photo or thermal oxidation. Such changes impair the recording characteristics of the recording layer.
One advantage of the present invention over those stabilizers (quenchers)of the prior art is the improved solubility and greater solution stability of formulations of cyanine dyes with aminium-type stabilizers and formulations of cyanine dyes with combinations of stabilizers in desirable coating solvents that do not attack polycarbonate.
Another advantage is the improved photostability of the dye recording layer, which can be achieved without altering the desirable absorption properties of the dye mixture at the recording wavelength.